1. Field of the Invention
This invention relates to a developing device generally applicable in electrophotographic copying machines, and more particularly to improvements in or relating to the magnetic brush developing device.
2. Description of the Prior Art
Among the developing devices heretofore applied in electrophotographic copying machines, there are known those which utilize the powder cloud method, the cascade method, the magnetic brush method or the like, and they have their own unique features and are pratically used in various fields in accordance with those features.
Among these, the powder cloud method and the cascade method are meritorious for the copying of business documents inasmuch as these have the edge effect so called because the developing toner concentrates upon surface portions of a photosensitive medium where the inclination of the electric field of an electrostatic latent image thereon is great, namely, the surface portions of the photosensitive medium which correspond to the portions of an original image in which the image density is discontinuous, and such portions appear emphasized in the resultant reproduced image. However, this same merit will present itself as a demerit in the reproduction of ordinary tone images, namely, the reproduction of the portions of an original image in which the image density is continuously varied. Further, any of the power cloud method and the cascade method is disadvantageous in that the developing devices therefor become larger in volume because of the requirement that the developing area must be wider.
In contrast, the magnetic brush method is such that a developer consisting of a mixture of iron powder and developing toner is attracted to a magnet so that the developer is formed into a brush-like shape at the portions of the magnet where the poles thereof exist, and the electrostatic latent image on the photosensitive medium is caused to make frictional contact with the brush-shaped developer to thereby accomplish development and reproduction of the image. In this case, the iron powder itself acts as a soft developing electrode and can therefore cause the developing toner to be deposited in proportion to the charge density of the electrostatic latent image and thus, this method is suitable for the reproduction of tone images. Also, the developing device for carrying out this method can be made compact.
As an improvement over such magnetic brush developing device, there is known a system which comprises a magnet rod disposed with the poles thereof arranged circumferentially of the rod so that adjacent ones of the successive poles are opposite in polarity, and a non-magnetic member provided around the magnet rod and in which the two members are moved relative to each other to thereby form magnetic brushes on the surface portions of the non-magnetic member corresponding to the magnetic poles so that these magnetic brushes make soft frictional contact with the surface to be developed, thereby accomplishing development. For example, refer to the following patents:
U.S. Pat. Nos. 3,040,704; 3,145,122; 3,152,924; 3,176,652; 3,455,276; 3,543,720; 3,608,522; 3,724,422; etc.
Such magnetic brush developing device according to the prior art has involved a difficult problem in the reproducibility of the density of original image. The reason is that once the toner, which is deposited on a copying medium, usually a sheet of paper, has covered the copying medium in the form of one or two layers, the density of the copy image is no longer varied but assumes saturated condition even if the number of the toner layers is increased. Because of this, the copy image will be very hard in visual impression as compared with the original image. In order to soften the impression of such copy image in an effort to approximate it to that of the original image, the quantity of toner deposited must generally be decreased, whereas this has in turn resulted in reduced reproducibility of character images.
Also, in the device of this type, an effort to utilize the repelling magnetic fields of the magnetic means has often encountered difficulties in transporting the magnetic developing material from the developing container to the developing station and there is known no method which perfectly utilizes the repelling magnetic fields as well as the coaction thereof with the centrifugal force, gravity, inertia force, etc. of the developing material.